1. Field of the Invention
This invention relates to an optical source module for generating light beams having different wavelengths and a manufacturing method thereof. Also, the invention is directed to an optical pickup apparatus which reads different types of optical recording media by employing the optical source module generating different wavelengths of beams.
2. Description of Related Art
In the recent development of recording media, the ability to record a higher capacity of information in accordance with the change into a large-scale of information quantity has become a requirement. Accordingly, an optical disc showing a remarkable increase of an occupation rate in the recording medium market also has been being developed such that it is possible to record large amounts of information. As a result, in the optical disc market, the digital versatile or video disc (hereinafter referred simply to as "CD") or the rewritable compact disc of write once read many (WORM) type (hereinafter referred simply to as "CD-R"). The DVD not only has a recording density than the CD and CD-R, that is, in the track density, but is also shorter than the CD and CD-R in a distance extending from the disc surface to the information recording face. In fact, the distance from the disc surface to the information recording face in the DVD is 0.6 mm while the distance in the CD and CD-R is 1.2 mm. Further, the information recording faces of the DVD and the CD-R have a different reflection ratio depending upon the wavelength of light beam incident thereon. Specifically, the reflection ratio in the information recording face of the CD-R is at a maximum value with a wavelength of 780 nm while the reflection ratio of the DVD is at a maximum value in the case of a wavelength of 650 nm. As described above, as optical discs having different structures and characteristics become commercially available, an optical pickup will need a capability of accessing all types of optical discs, such as CD, CD-R and DVD.
In order to satisfy this need, an optical pickup apparatus adopting a two beam system which employs two light sources has been suggested. As shown in FIG. 1, the two beam system optical pickup apparatus comprises first and second light sources 10 and 12 which are separately installed for generating light beams of 650 nm and 780 nm, respectively, and first beam splitter 14 for matching paths of the light beams from light sources 10 and 12. The first light source 10 generates a light beam having a wavelength of 650 nm (hereinafter referred to as first light beam B1) when accessing a DVD 11, and supplies first light beam B1 via first collimator lens 16 to first beam splitter 14. On the other hand, the second light source 12 generates a light beam with a wavelength of 780 nm (hereinafter referred to as the second light beam B2) when accessing a CD or CD-R 13, and supplies second light beam B2 via second collimator lens 18 to first beam splitter 14. Herein, the first light source 10 is arranged in an optional position making a horizontal line with respect to the first beam splitter 14 (for example, the right side of FIG. 1) while the second light source 12 is arranged in an optional position making a vertical line with respect to the first beam splitter 14 (for example, the upper side of FIG. 1), respectively, in such a manner that respective paths of first light beam B1 and second light beam B2 make a right angle with respect to each other. Accordingly, first beam splitter 14 transmits the first light beam B1 as is, while it reflects the second light beam B2 at a right angle thereto, to thereby match the path of the first light beam B1 with that of the second light beam B2. The first light beam B1 from the first beam splitter 14 is converged onto an information recording face 11A, by way of second beam splitter 20, a right angle reflective glass 22, and an objective lens 24, in turn, in the shape of spot. Subsequently, the light beam reflected by an information recording face 11A of the DVD 11 arrives at the surface of a multiple divisional photo detector 28 by way of the object lens 24, the right angle reflective glass 22, the second beam splitter 20, and a sensor lens 26, in turn. In a similar manner, the second light beam B2 from the first beam splitter 14 is converged onto an information recording face 13A of the CD or CD-R 13, via the second beam splitter 20, the right angle reflective glass 22 and the objective lens 24, in turn, in a shape of spot, and is thereafter reflected by the information recording face 13A of the CD or CD-R 13. Further, this reflected light beam progresses toward the multi-divisional photo detector 28 by way of the objective lens 24, the right angle reflective glass 22, the second beam splitter 20 and the sensor lens 26, in turn. Accordingly, the multi-divisional photo detector 28 converts the light beam incident thereto from the sensor lens 26 into an electrical signal. This electrical signal includes information recorded on the CD or CD-R 13, or the DVD 11.
The above-described optical pickup apparatus using a two beam system, however, requires additional optical devices in order to align the progressive paths of the light beams from the respective light sources. Because of this, the two beam system optical pickup apparatus has disadvantages in that it has a complicated configuration as well as being large bulky. To overcome these disadvantages of the two beam system optical pickup apparatus, various optical pickup apparatuses such as holographic, liquid crystal shutter, and annular shield have been devised, which are capable of accessing CD, CD-R and DVD discs utilizing a single light beam. Firstly, the holographic optical pickup apparatus allows two diffractive light beams different in flux diameter to be incident on the objective lens by utilizing the hologram lens, thereby forming a focus onto all the information recording faces of the CD or CD-R and the DVD. Secondly, the liquid crystal shutter optical pickup apparatus controls the flux diameter of the light beam incident on the objective lens by utilizing a liquid crystal plate and a polarizer plate, thereby converging the light beam onto the information recording face of the CD or CD-R, or the DVD. Finally, the annular shield type of optical pickup apparatus converges the light beam onto all of the recording faces of the CD or CD-R and DVD in such a manner that a side lobe is not generated by utilizing an objective lens having an annular band formed in the surface thereof.
These optical pickup apparatuses each can converge the light beam onto the information recording faces of each of the CD, CD-R and DVD by changing a focus distance of the objective lens, but cannot read out the information recorded on the CD-R. This is because the reflection ratios in the information recording face of the CD-R and the information recording face of the DVD are different depending upon a wavelength of light beam. In other words, the reflection ratio in the information recording face of the CD-R has a maximum value at a wavelength of 650 nm. This results in the conventional holographic, liquid crystal shutter, and annular shield optical pickup apparatuses being not compatible with each of the CD, CD-R and DVD.